U.S. patent number 6,193,480 [Application Number 09/128,302] was granted by the patent office on 2001-02-27 for system and method for increased flow uniformity.
This patent grant is currently assigned to Alaris Medical Systems, Inc.. Invention is credited to Robert D. Butterfield.
United States Patent |
6,193,480 |
Butterfield |
February 27, 2001 |
System and method for increased flow uniformity
Abstract
A system for increasing the volumetric flow uniformity of fluid
pumped through a conduit by an infusion pump. A pumping mechanism
operates in identifiable step movements that are broken down into
microstep movements that are then grouped into packets. The pumping
mechanism controls the period of each microstep, so that the sum of
the microstep periods in a packet is essentially equal to the
packet period, with little or no waiting time on the motor. The
motor preferably begins moving the assigned microsteps in each
packet immediately upon the beginning of the time period, but
controls the period of the microsteps for that packet.
Inventors: |
Butterfield; Robert D. (Poway,
CA) |
Assignee: |
Alaris Medical Systems, Inc.
(San Diego, CA)
|
Family
ID: |
22434665 |
Appl.
No.: |
09/128,302 |
Filed: |
August 3, 1998 |
Current U.S.
Class: |
417/477.1;
417/43 |
Current CPC
Class: |
A61M
5/16831 (20130101); F04B 43/082 (20130101); F04B
2203/0213 (20130101) |
Current International
Class: |
A61M
5/168 (20060101); F04B 43/00 (20060101); F04B
43/08 (20060101); F04B 043/08 (); F04B
043/12 () |
Field of
Search: |
;417/477.1,43,53,12,44.1,45,300 ;318/685 ;604/153 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Yuen; Henry C.
Assistant Examiner: Gimie; Mahmoud M
Attorney, Agent or Firm: Fulwider Patton Lee & Utecht,
LLP
Claims
What is claimed is:
1. A system for increasing the uniformity of flow of fluid through
a conduit comprising:
a drive device that moves in movement increments;
a pumping mechanism coupled to the drive device that pumps fluid
through the conduit in volume increments in response to movement
increments of the drive device, with each volume increment
corresponding to a respective movement of the drive device, at
least some of the volume increments differing from other volume
increments; and
a processor configured to:
group the movement increments into movement groups as a function of
the volume increments corresponding to the respective movement
increments, with each movement group having a total flow volume
equal to the combined volume increments of the movement increments
in the movement group, wherein the total flow volumes of all
movement groups are approximately equal,
assign a group period to each movement group, wherein the group
periods of all movement groups are equal;
determine an increment period for each movement increment, wherein
the combined increment periods of all movement increments in a
movement group are approximately equal to the group period for the
movement group; and
control the drive device to move in the movement increments, with
the movement increments occurring at the assigned increment
periods.
2. The apparatus of claim 1, wherein the processor is configured to
control the drive device to cause the movement increment period of
each movement increment within an increment group to be
proportional to the fluid flow volume increment of the movement
increment.
3. The apparatus of claim 1, wherein the processor is configured to
control the drive device to cause the movement increment period of
all movement increments within an increment group to be equal.
4. The apparatus of claim 1, further comprising:
an input device that provides a desired flow rate value, and
wherein the processor is configured to receive the desired flow
rate value, and to determine the length of each group period as a
function of the desired flow rate value.
5. The apparatus of claim 1, further comprising:
an input device that provides a desired flow rate value, and
wherein the processor is configured to receive the desired flow
rate value, and to group the movement increments into movement
groups as a function of the desired flow rate value.
6. A parenteral fluid delivery system comprising:
a fluid delivery conduit;
a peristaltic pump, said pump having a plurality of peristaltic
elements movable into and out of engagement with the conduit to
control the flow of fluid through the conduit;
a drive device having a plurality of positions through which the
drive device may move in incremental movements, said drive device
coupled to the peristaltic elements of the pump, wherein the
peristaltic elements pump fluid through the conduit in volume
increments in response to incremental movements of the drive
device, each volume increment corresponding to a respective
incremental movement of the drive device;
a position sensor that monitors the position of the drive
device;
a memory that is configured to store data representing the volume
increment of fluid flow corresponding to each incremental movement
of the drive device; and
a processor in communication with the memory, said processor
configured to:
receive the drive device position signal and the fluid flow volume
increment data,
assign incremental movements of the drive motor to incremental
movement groups as a function of the fluid flow volume increment
data for each incremental movement, with the combined fluid flow
volume increments of all incremental movements within a movement
group approximately equal to the combined fluid flow volume
increments of other movement groups,
assign a timeslot for each incremental movement group, with all
timeslots having an equal period, and
control the drive device to cause the movement increments in each
movement group to be spread across the timeslot period for that
movement group, with each movement increment having a period, with
the sum of all movement increment periods in a movement group being
equal to the movement group period.
7. The apparatus of claim 6, wherein the processor is configured to
control the drive device to cause the movement increment period of
each movement increment within an increment group to be
proportional to the fluid flow volume increment of the movement
increment.
8. The apparatus of claim 6, wherein the processor is configured to
control the drive device to cause the movement increment period of
all movement increments within an increment group to be equal.
9. The apparatus of claim 6, wherein the processor is configured to
move through a predetermined number of movement increments without
delay in the event that a volume increment for a respective
incremental movement is negative.
10. The apparatus of claim 6, wherein the drive device comprises a
cam shaft rotated by a step motor, said step motor moving in a
series of motor steps, with each motor step corresponding to a
movement increment.
11. The apparatus of claim 6, wherein the drive device comprises a
cam shaft rotated by a step motor, said step motor moving in a
series of motor steps, with said motor steps further divided into
microsteps, with each microstep corresponding to a movement
increment.
12. A method for providing more uniform flow of fluid through a
conduit acted upon by a pumping mechanism, the pumping mechanism
moving in movement increments with each movement increment causing
a particular fluid flow volume increment through the conduit, the
method comprising:
determining a desired fluid delivery flow rate;
assigning movement increments to movement groups as a function of
the fluid flow volume increment of each movement increment, with
the combined fluid flow volume increments of all movement
increments in a movement group being approximately equal to the
combined fluid flow volume increments of all movement increments in
other movement groups;
assigning a timeslot for each incremental movement group as a
function of the desired fluid delivery flow rate, with all
timeslots having an equal period;
controlling the pumping mechanism to cause the movement increments
in each movement group to be spread across the timeslot period for
that movement group, with each movement increment having an
increment period, with the sum of all movement increment periods in
a movement group being equal to the movement group period.
13. The method of claim 12, wherein the step of controlling the
pumping mechanism includes controlling the pumping mechanism to
cause the movement increment period of each movement increment
within an increment group to be proportional to the fluid flow
volume increment of the movement increment.
14. The method of claim 12, wherein the step of controlling the
pumping mechanism includes controlling the pumping mechanism to
cause the movement increment period of all movement increments
within an increment group to be equal.
15. The method of claim 12, wherein the pumping mechanism comprises
a peristaltic mechanism driven by a step motor, said step motor
moving in a series of steps, and each motor step corresponds to a
movement increment of the pumping mechanism.
16. The method of claim 12, wherein the pumping mechanism comprises
a peristaltic mechanism driven by a step motor, said step motor
moving in a series of steps, and the method includes the further
step of:
dividing each motor step into a plurality of microsteps, wherein
each microstep corresponds to a movement increment of the pumping
mechanism.
Description
BACKGROUND
The present invention relates generally to a system and method for
controlling the flow of fluids through a conduit, and in particular
to controlling a pump acting on a conduit for increasing the
uniformity of the fluid flow through the conduit.
In certain systems used for infusing parenteral fluids
intravenously to a patient, a pumping mechanism engages a length of
conduit or tubing of a flexible administration set to pump the
parenteral fluid to the patient at a selected flow rate. A
peristaltic pump is one commonly used type of pumping mechanism and
employs the sequential occlusion of the administration set tubing
to move the fluid through the tubing to the patient.
Linear-type peristaltic pumps typically include a row of adjacent,
reciprocating pumping fingers that are sequentially urged against
the fluid administration set tubing to occlude adjacent segments of
that tubing in a wave-like action to force fluid through the
tubing. The reciprocating, sequential motion of the fingers is
accomplished in one arrangement by the use of a cam shaft rotated
by a drive motor. Disposed along the length of the cam shaft are a
plurality of adjacent cams having generally symmetrical lobe
geometries with one cam operating each finger. The cams are
disposed along the cam shaft so that adjacent lobes project at
different angular positions relative to the cam shaft. The fingers
in turn advance and retract sequentially in accordance with the
angular positions of the respective cam lobes and rotation of the
cam shaft.
The drive motor typically comprises a step motor having a certain
number of motor steps per complete rotation of its armature; for
example, two-hundred steps per 360 degrees of rotation. Typically,
a pump cycle is defined as a complete cycle of the pumping
mechanism. For example, in the case of a twelve-finger linear
peristaltic pump, a pump cycle is complete when all twelve fingers
have engaged the fluid conduit and returned to the positions they
had at the start of the cycle. In many such systems, when the pump
mechanism has completed a full cycle the step motor will have also
traveled through 360 degrees of rotation, thereby causing it to
have travelled through all of its steps in that rotation.
Each incremental movement of the motor causes a corresponding
incremental movement of the cams and fingers and results in a
discrete volume of fluid or "step volume" being pumped through the
conduit. An inherent characteristic of linear peristaltic pumps is
that step volumes vary from other step to step, and at certain
points over a pump cycle the step volume may even be negative
(i.e., reverse flow). This reverse flow results when the outlet
side fingers of the linear peristaltic pump are retracted from the
tubing and a reverse flow surge backfills the tubing pumping
segment due to a pressure difference between the pumping segment
and the downstream segment.
In one effort to increase the flow uniformity within a peristaltic
pump cycle, the design of the pumping mechanism was tailored. For
example, tailored, non-symmetrical cam lobes have been developed to
accelerate, decelerate or limit the advancement of the pumping
fingers as they engage and disengage segments of the tubing. Some
of these designs have resulted in increased uniformity of volumes
pumped per motor step at a particular design flow rate. However, it
has been found that the effectiveness of these designs decreases at
flow rates that differ significantly from the design flow rate.
Another approach to increasing flow uniformity is described in U.S.
Pat. No. 5,716,194 to Butterfield et al., entitled SYSTEM FOR
INCREASING FLOW UNIFORMITY, the contents of which are incorporated
herein by reference. In U.S. Pat. No. 5,716,194, flow uniformity
was enhanced by grouping several adjacent steps into larger
"supersteps," with each superstep comprised of a group of steps. By
carefully grouping of the steps, supersteps can be created in such
a way that each superstep has essentially the same volume of fluid
as the other supersteps. For example, one superstep may consist of
7 relatively low-volume motor steps, while another superstep may
consist of 3 larger-volume motor steps. By associating more of the
low-volume motor steps on the first superstep, the total volume of
the first superstep approximately equals the total volume of the
second superstep. With supersteps of generally equal volume and
period, flow uniformity is enhanced.
For lower flow rates, the use of such supersteps can require long
pauses in pump operation between the steps. A single motor step
may, for example, produce a bolus of fluid which, to produce flow
at the desired flow rate, requires substantial time to elapse
before the next motor step occurs. Moreover, in some cases, even
with long pauses between steps, a particularly large-volume step
may cause the system to momentarily exceed the desired flow rate.
The problem of such large-volume steps could be increased by the
use of supersteps that consist of more than one step.
Various modifications to fluid pump drive systems can be made to
address uniformity at low flow rates, including the addition of a
gear train and/or development of a pump having a greater number of
steps per revolution. Such modifications can, however, be expensive
in that they typically require development of an entirely new pump
mechanism.
In part to address concerns for low flow rates, a motor drive
technique known as "microstepping" was developed, wherein each
motor step was subdivided into a series of smaller microsteps. For
example, each motor step might be subdivided into up to eight
different microsteps. Those microsteps could then be grouped into
"packets" of microsteps, with each packet having essentially the
same volume as other packets.
Microstepping has been found to increase flow uniformity and
significantly reduce motor noise. Microstepping involves driving
the step motor through a step with a series of current magnitude
states that generate small angular displacements of the field
vector position. The sum of these displacements equals that of one
step. Because instantaneous torque is approximately a sinusoidal
function of angular displacement of a motor's field vector position
from its rotor position, a smaller angular displacement results in
a lower instantaneous torque. A lower instantaneous torque
generates an angular acceleration at the leading edge of each
"microstep" smaller than that generated at the leading edge of each
step in "full step" drive mode. The effect is to spread the large
acceleration that normally occurs at the beginning of a step over
the entire step as a series of small accelerations, thus reducing
the level of acoustic noise. Thus, rather than turning through an
entire step in near-instantaneous fashion, the motor can instead
moves through a series of distinct incremental microsteps, each of
which involves only a portion of the movement turn of an entire
step.
Several existing systems make use of microsteps in various drive
motors, including fluid pump motors. For example, U.S. patent
application Ser. No. 08/526,468 to Holdaway, entitled "OPEN-LOOP
STEP MOTOR CONTROL SYSTEM," which is incorporated herein by
reference in its entirety, describes using microsteps in driving an
infusion pump step motor.
In existing implementations, the duration of each microstep was
typically fixed at a nominal value, such as 2.36 milliseconds. An
entire packet of microsteps would often be made in relatively rapid
succession, followed by a "non-flow time" during which no motor
movement would occur. The average flowrate was adjusted by reducing
or increasing the volume in the packets (i.e., by adjusting the
number of microsteps in each packets), and also by adjusting the
non-flow time (i.e., the time between microsteps in which the motor
was not moving).
The non-flow period could be actively varied in order to change the
average flow rate as well as to enhance other system functions. For
example, U.S. patent application Ser. No. 08/688,698 to
Butterfield, entitled FLUID FLOW RESISTANCE MONITORING SYSTEM,
which is incorporated herein by reference in its entirety,
describes a system that varies fluid delivery, including non-flow
periods, using a pseudorandom code. For very low flow rates, the
non-flow time might become relatively large. For example, a desired
flow rate of 0.1 ml/hr might involve a non-flow period on the order
of 200 seconds.
In fluid driving systems, there are circumstances wherein maximum
flow uniformity is desirable. For example, in parenteral infusion
of some fluids that require very low flow rates, such as certain
fast acting (i.e., short half-life) drugs, it can be desirable to
maintain minimal fluctuation of the instantaneous flow rate. This
need for minimal fluctuation of the flow rate can become most acute
in the lower ranges of flow typically produced by commercial
peristaltic infusion devices, such as the range from 0.1 to 1.0
ml/hr.
Some organizations, such as the Emergency Care Research Institute
(ECRI), have promulgated ratings of flow uniformity based on the
interval between "flow steps" at the lowest flow rate achieved.
Such ratings, although typically somewhat indefinite, can provide
useful guidelines. For example, ECRI rates an infusion pumps flow
uniformity as "excellent" if less than 20 seconds elapse between
"flow steps" at the "lowest rate programmable." Assuming that the
ECRI rating is based on having steps of equal volume, many current
commercial devices are far from meeting such criteria.
Hence those skilled in the art have recognized the need for
increasing flow uniformity, particularly at low flow rates. The
present invention fulfills these needs and others.
SUMMARY OF THE INVENTION
Briefly, and in general terms, the present invention provides a
system and method for controlling the flow of fluid in a conduit
acted on by a pumping mechanism by controlling the movement of the
pumping mechanism to obtain increased flow uniformity. In one
aspect, a system for controlling the flow of fluid through a
conduit in response to a selected flow rate to provide more uniform
flow is provided wherein the system comprises a pumping mechanism
acting on the conduit to control the flow of the fluid through the
conduit, the pumping mechanism including a plurality of pumping
devices that compress the conduit in a predetermined pumping
pattern to cause fluid movement through the conduit, the mechanism
moving in successive steps of movement of the pumping devices
through a complete pumping cycle. Included is a memory in which is
stored a quantity of fluid that flows through the conduit
corresponding to each movement step of the pumping mechanism; and a
processor is adapted to select and group successive steps of
movement of the pumping devices in packets to pump as close to the
target flow volume as possible in each packet of steps; the
processor being further adapted divide the steps into microsteps
and to cause the pump motor to drive through the microsteps so as
to having within each packet microsteps with equal periods to the
other microstep periods in that packet.
In a further aspect, the pumping mechanism moves through the
microsteps assigned to each packet during all or substantially all
of the packet time period, so that the waiting period during which
the mechanism does not move is minimized or eliminated.
In a more detailed aspect, the pumping mechanism comprises a step
motor driving the pumping devices into and out of contact with the
fluid conduit in the predetermined pattern to cause fluid to flow
through the conduit, the memory stores a volume of fluid flow
through the conduit that corresponds to each step of the step
motor, wherein the processor controls the step motor to move in
movement microsteps having microstep periods that are determined as
a function of the flow volume for the microstep period and in the
particular packet.
In yet another aspect, the processor selects the microstep period
based on the flow rate, with the length of the microstep period
selected being inversely proportional to the flow rate
selected.
In yet another aspect, the pumping mechanism passes at high or
maximum speed through a series of pump steps during which the sum
total flow is essentially zero.
Other features and advantages of the invention will become apparent
from the following detailed description, taken in conjunction with
the accompanying drawings, which illustrate, by way of example, the
features of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a polar graph of the volume pumped per motor step over
one pump cycle of a common linear peristaltic pump;
FIG. 2 is a linear graphical representation of the pump cycle shown
in FIG. 1;
FIG. 3 is a graphical representation of fluid flow superimposed
over packet time periods with a substantial waiting period;
FIG. 4 is a graphical representation of fluid flow superimposed
over packet time periods where the waiting period is relatively
small; and
FIG. 5 is a schematic illustration of a linear peristaltic fluid
delivery system embodying features of the invention and employing a
position sensor and step motor under processor control to move
parenteral fluid from a fluid reservoir to a patient.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings with more particularity, wherein like
reference numerals in the separate views indicate like or
corresponding elements, there is shown in FIGS. 1 and 2 a pump
cycle flow pattern for a typical linear peristaltic pump, with the
pump having a motor cycle of 200 steps. FIG. 1 shows a polar graph
of the pump cycle flow pattern, while FIG. 2 shows a linear graph
of the same pattern.
In FIG. 1, individual motor steps 10 are shown beginning at a
reference point "0" and sequentially move in equal angular
increments in a clockwise direction over a complete 200-step
rotation returning to the reference point "0". The pump cycle flow
pattern 12 results. Zero flow is represented by a circle 14,
positive flow 16 is represented outside the circle, and negative
flow 18 is represented inside the circle. A net reverse or negative
flow period is illustrated by the arc 20, with a net zero flow
period illustrated by the arc 21. By referring to the portion of
the pattern corresponding to a particular motor step, the volume
pumped by that step (step volume) can be determined. Step volumes
can be determined by means well known to those skilled in the art,
such as by gravimetric measurement.
FIG. 2 presents the same data as FIG. 1 except in a linear
graphical format. Individual motor steps 10 are shown beginning at
a reference point "0" defined at the intersection of the X and Y
axes and each subsequent motor step is represented along the
X-axis. The pump cycle flow pattern 12 resulting from the
individual step volumes pumped corresponding to each motor step
rotated is shown. Zero flow is represented by the X-axis 22,
positive flow is represented by the positive Y-axis 24 and negative
flow is represented by the negative Y-axis 26. A net reverse or
negative flow period 28 is also illustrated. As is apparent from an
observation of both FIGS. 1 and 2, different volumes are pumped per
step during the pump cycle 30.
Increased flow uniformity can be achieved by dividing motor steps
into microsteps and then grouping those microsteps into "packets"
having equal periods T.sub.P and generally equal fluid volumes
Q.sub.P, as set forth in FIG. 3. In the embodiment depicted in FIG.
3, three adjacent packets are composed of four, five, and two
microsteps 32, respectively. The area under each microstep
corresponds to the fluid flow volume Q.sub.MS for that microstep,
with the sum of the fluid flow volumes within each packet (i.e.,
Q.sub.P) being approximately the same as the total volume for the
other packets.
Note that the grouping or "packetizing" may actually be conducted
at the step level, whereby different steps are assigned to packets,
and the steps are then broken into smaller microsteps within the
packet. Because different steps and microsteps can have different
volumes Q.sub.S, Q.sub.MS, the number of steps and microsteps can
vary from packet to packet to maintain generally equal packet fluid
volumes Q.sub.P. The packet volume Q.sub.P for a particular packet
is thus defined as the sum of all step and/or microstep volumes in
that packet, as follows:
Because of variations in the step and microstep volumes, the packet
volume is usually not precisely equal from packet to packet.
However, by carefully selecting and grouping the steps and/or
microsteps for each packet, the packet volume Q.sub.P can be held
relatively constant from packet to packet, even where the step and
microstep volumes Q.sub.S, Q.sub.MS vary widely.
In one embodiment of the invention, packets are assigned at the
step level, and the steps are then divided into microsteps. In a
further embodiment, different steps may be divided into different
numbers of microsteps. For example, the first step in a packet may
be divided into an initial large number of microsteps, the second
divided into half as many microsteps, the third into one-fourth as
many microsteps, and so on, with the number of microsteps per step
decreasing until a single microstep per step is used. At the end of
the packet, the process is reversed, with the third-to-last step
being divided into the same number of steps as the third step, the
second-to-last being divided into the same number of microsteps as
was the second step, and the last step being divided into the same
initial large number as was the first step. As an example of such
an embodiment, in a packet of 9 steps, the steps are broken into
microsteps as set forth in Table A:
NUMBER OF CORRESPONDING STEP # MICROSTEPS 1 8 2 4 3 2 4 1 5 1 6 1 7
2 8 4 9 8
Note that the particular embodiment depicted in Table A has a set
limit of eight for the number of microsteps into which any step can
be divided.
In another embodiment, each step may be divided into the same
number of microsteps. For example, all steps might be divided into
4 microsteps.
In typical infusion systems, the pump motor rapidly advances
through each microstep, so that microsteps 32 each have fixed,
generally identical (and relatively small) periods T.sub.MS, as
shown by way of example in FIG. 3. Microstep periods for typical
pumps are on the order of just a few milliseconds. Thus, in order
to achieve the desired flow rate while maintaining a generally
constant packet period T.sub.P, a relatively large waiting period
of pump inactivity T.sub.W can be used to extend the packet period
T.sub.P to the desired value. In such systems, the waiting period
T.sub.W varies from packet to packet, depending on the number of
microsteps in the packet. As was discussed previously, for low flow
rates the waiting period T.sub.W can become quite large when the
microstep periods T.sub.MS are small. For example, for a flow rate
of 0.1 ml/hr can involve a waiting period T.sub.W (i.e., non-flow
period) on the order of 200 seconds.
As depicted in FIG. 4, the current invention eliminates or
minimizes the waiting period T.sub.W by enlarging the individual
microstep periods T.sub.MS. By controlling the pump motor to very
slowly pass through the microsteps, the microstep periods T.sub.MS
are extended so that, in total within a packet, they encompass all
or substantially all of the packet period T.sub.P, thereby
eliminating or at least minimizing the waiting period T.sub.W. Note
that the motor may not actually be moving during the entirety of
each microstep period T.sub.MS. Due to the mechanical and
electrical characteristics and behavior of step motors, microstep
movement is usually not consistent throughout the microstep period
T.sub.MS. For example, as depicted in FIG. 4a, the microstep may
involve substantial motor movement at the very beginning of the
microstep period T.sub.MS, with that movement slowing down
afterward so that toward the end of the microstep period T.sub.MS
there may in fact be little or no movement of the motor. This time
of non-movement is generally small, however, and does not create
substantial non-flow times.
In an embodiment of the invention, the waiting period T.sub.W at
the end of each packet is not completely eliminated, but is instead
brought down to a very low value. In a further embodiment, the
waiting period T.sub.W is held generally constant, preferably at a
very low value, from packet to packet. This is in contrast to
systems that hold the step or microstep period T.sub.MS constant
and instead vary the waiting period T.sub.W to achieve desired flow
rates.
In the particular embodiment of FIG. 4, the waiting period T.sub.W
is so small as to almost negligible. The microstep periods T.sub.MS
are generally constant within each particular packet, with the
microstep periods T.sub.MS in a packet generally equal to the
packet period T.sub.P divided by the number of microsteps in that
period. For example, in Packet 1 from FIG. 4, there are 4
microsteps, so that each microstep period T.sub.MS1 is equal to or
about 1/4 of the total packet period T.sub.P. For packet 2, which
has 5 microsteps, the microstep period T.sub.MS2 is equal to about
1/5 of the total packet period T.sub.P. Thus, the microstep period
is generally defined as follows:
where n.sub.Ms is the number of microsteps in the packet.
In an alternate embodiment, the microstep period could be
determined for each microstep individually, possibly taking into
account the volume delivered in the particular microstep. As an
example, the microstep period T.sub.MS may be a function of the
total packet period T.sub.P, the total volume delivered in the
packet Q.sub.P, and the volume delivered by the particular
microstep Q.sub.MS. An equation such as the following might be
employed:
Information regarding the volumes Q.sub.MS and/or Q.sub.P might be
held in a table that the system processor consults for the various
steps and microsteps.
To increase the flow rate, the system can increase the number of
microsteps in each packet (thereby increasing the packet volume
Q.sub.P). Alternatively (or additionally), the system can increase
the flow rate by decreasing the packet period T.sub.P. To decrease
the flow rate, the system can decrease the number of microsteps per
packet and/or increase the packet period T.sub.P.
In a preferred embodiment of the invention, the packet volume
Q.sub.P (and therefore the number of microsteps) in each packet is
held at a generally constant value for all flow rates in specific
range. In a more specific embodiment, the packet volume Q.sub.P is
maintained at the 5-6 .mu.l level for flow rates under 50 ml/hr. By
maintaining the packet volume Q.sub.P generally constant, the
system does not have to redetermine the appropriate number of
microsteps for each packet every time the flow rate is changed.
In a typical pump mechanism having 200 steps per revolution, one
revolution of the pump mechanism usually delivers in the range of
about 165 to 200 microliters. Thus, it takes about five or so
complete revolutions of the pump mechanism to deliver a milliliter
of fluid. These five or so complete revolutions involve over 1000
steps, so that delivery of a milliliter of fluid is spread across
over 1000 steps. However, due to the physical characteristics and
operation of most peristaltic devices, a significant number of
back-to-back steps in each revolution do not, in total, produce
significant fluid flow. For example, in the pump cycle flow pattern
depicted in FIG. 1, the arc identified as 21 produces, in total,
essentially zero fluid flow. Rather than try to apportion these
series of low or even negative flow steps out across various
packets, it has been found to be effective to simply run the motor
at maximum or relatively high speed through the series of steps in
arc 21. For example, the series of steps in the zero sum fluid flow
arc might be traversed by the pump in a very short time, on the
order of 100 milliseconds. Because the sum total of the flow in the
series of steps in arc 21 is zero, such a rapid advancement through
those steps helps to enhance the flow uniformity.
FIG. 5 is a block diagram of a fluid delivery system 40
incorporating aspects of the current invention. The fluid delivery
system includes a fluid delivery conduit 42 acted upon by a pumping
mechanism 46 driven by a pump motor 44. In the embodiment shown,
the pumping mechanism 46 comprises a rotating cam shaft 48 coupled
to the pump motor 44 and moving a series of peristaltic elements
50. The peristaltic elements 50 operate on the conduit 42 to move
fluid from a fluid source 52, through the conduit 42, and into a
patient 54 via a cannula 56.
A user input device 58, such as a keypad, provides operator
instructions, such as flow rate selection, to a processor 60. The
processor 60 controls the operation of the pump motor 44 driving
the pumping mechanism 46. A motor position sensor 62 determines the
position of the motor 44 and pumping mechanism 46 and provides a
position signal to the processor 60. A memory 64 may be provided to
store and provide appropriate information, such as tables of
information relating to volume per step and/or microstep.
The system may include various elements for monitoring system
parameters, such as those set forth in pending U.S. patent
application Ser. Nos. 08/688,698 and 08/526,468. For example, the
system may include a pressure sensor 66 coupled to the conduit 42
to sense pressure in the conduit. An analog-to-digital converter 68
("A-to-D") receives the analog pressure output signals from the
sensor 66 and converts them to a digital format at a particular
sample rate controlled by the processor 60. The processor 60
receives the digital pressure signals, processes them as described
in more detail below and calculates the resistance to flow. A
display 70 may be included to present system information, such as
resistance or flow rate, and one or more alarms 72 may be provided
to indicate an unsatisfactory operational parameter.
In the embodiment depicted in FIG. 5, the selection of a flow rate
is made at the keypad 58 and is received by the processor 60. The
user may also select at the keypad 58 between operational modes,
such as variable pressure mode (for low flows where high flow
uniformity is critical) and resistance monitoring mode (such as the
resistance monitoring mode described in pending U.S. patent
application Ser. No. 08/688,698).
A fluid delivery system in accordance with the current invention
may include one or more modes of operation, with use of the
increased flow uniformity elements and method steps only allowed
during certain of the operational modes. For example, a fluid
delivery system may have a first mode, such as a resistance
monitoring mode, during which monitoring of system parameters such
as resistance might be interfered with by the uniformity elements
and methods. Accordingly, the system might prohibit use of the
heightened flow uniformity elements and methods during operation in
the first mode. Such a fluid delivery system may have a second
mode, such as a variable pressure mode, during which the system
permits operation of the uniformity elements and methods.
In a preferred embodiment, the system will not allow the user to
select incompatible combinations of flow rates and operational
modes. For example, in a particular embodiment, the use of a
heightened flow uniformity mode may be restricted to flow rates of
50 ml/hr or less. Accordingly, the system would not allow the user
to select a combination of a heightened flow uniformity mode with a
flow rate over 50 ml/hr.
While the invention has been illustrated and described in terms of
certain preferred embodiments, it is clear that the invention can
be subject to numerous modifications and adaptations within the
ability of those skilled in the art. Thus, it should be understood
that various changes in form, detail and usage of the present
invention may be made without departing from the spirit and scope
of the invention.
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